High temperature, solid electrolyte, electrochemical generators employing interconnected electrochemical fuel cells convert chemical energy into direct current electrical energy at temperatures of about 800.degree. C. to 1200.degree. C. Such solid electrolyte fuel cells and multi-cell generators have been discussed in U.S. Pat. No. 4,395,468 (Isenberg). Fuel electrode, air electrode, solid electrolyte, and interconnection configurations are taught in U.S. Pat. No. 4,490,444 (Isenberg). Each electrochemical fuel cell typically includes a porous support tube (optional) made of, e.g., calcia stabilized zirconia, and about 1 to 2 mm thick. A porous air electrode or cathode is deposited on and generally surrounds the support tube made of, e.g., lanthanum manganite (LaMnO.sub.3), and about 0.05 to 1.5 mm thick. A dense, gas-tight, solid electrolyte is deposited on and substantially surrounds the outer periphery of the air electrode made of, e.g., yttria stabilized zirconia ((ZrO.sub.2).sub.0.9 (Y.sub.2 O.sub.3).sub.0.1), and about 0.001 to 0.1 mm thick. A porous fuel electrode or anode is deposited on and substantially surrounds the outer periphery of the solid electrolyte made of, e.g., nickel-zirconia cermet or cobalt-zirconia cermet, and about 0.1 mm thick. Both the solid electrolyte and the fuel electrode are discontinuous to allow for inclusion of the interconnect on the air electrode to provide means to electrically connect adjacent electrochemical fuel cells. A dense, gas-tight, interconnect is deposited on a selected radial segment of the air electrode, at the portion that is discontinuous in the electrolyte and fuel electrode, made of calcium, strontium, or magnesium doped lanthanum chromite (LaCrO.sub.3), and about 0.03 mm to 0.1 mm thick. A top layer is deposited over the interconnect made of nickel-zirconia cermet or cobalt-zirconia cermet, and about 0.1 mm thick. In multi-cell solid electrolyte electrochemical generators, the individual cells are connected at least in series through the electrically conducting interconnect with remains exposed to both fuel and oxidant gases.
Various air electrode compositions are taught in U.S. Pat. Nos. 4,562,124 (Ruka); 4,751,152 (Zymboly); 4,888,254 (Reichner); and, 5,108,850 (Carlson, et al.). The air electrode as taught are comprised of doped, e.g., strontium, calcium, barium, cerium, and/or chromium doped lanthanum manganite materials which have compatible chemical and thermal expansion properties with the electrolyte and interconnect materials. The air electrode materials that are presently being used are doped lanthanum manganite compositions, the nature and level of doping being selected on the basis of improvements in the electrode conductivity and structural stability of the air electrode. Various methods have been used to apply both the electrolyte and interconnect material to the top of the air electrode. Conventionally, both the electrolyte and the interconnect material are applied to the surface of different selected portions of the air electrode by a modified electrochemical vapor deposition process at temperatures of about 1200.degree. C. to 1400.degree. C., employing the use of vaporized halides of zirconium and yttrium for the electrolyte and of lanthanum, chromium, magnesium, calcium or strontium for the interconnect for deposition on the air electrode as taught in U.S. Pat. Nos. 4,597,170 (Isenberg) and 4,609,562 (Isenberg, et al.).
Such halide vapors can interact with and degrade the air electrode material and adjacent interfaces during the initial period of electrolyte and interconnect application. This can cause, in some instances, leaching of air electrode constituents, e.g. lanthanum, manganese, calcium, etc. Leaching of the air electrode constituents accordingly results in alteration of electrical, chemical and mechanical properties of the air electrode, due to substantial modification at the electrolyte as well as at the interconnect interface. Interconnection layer applied by other techniques are also liable to degrade with time due to continuing interaction with the air electrode. Additionally, even after electrolyte and interconnect application, there may be long term diffusion of manganese from the air electrode into the interconnect during operation of the electrochemical fuel cell, which accordingly results in alteration of electrical, chemical and mechanical properties of the interconnect interface and, consequently, reduces the life of the electrochemical cells.
During prolonged exposure of electrochemical cells to elevated operating temperatures of about 1000.degree. C., it has been observed that the interconnect (i.e,, doped LaCrO.sub.3) in contact with the air electrode (i.e., doped LaMnO.sub.3) undergoes structural changes. Void formation and second phase precipitates comprised of Mn--Cr oxides have been identified which mostly occur at the grain boundary of the interconnect. Manganese diffusion from the air electrode at the air electrode-interconnect interfacial boundary accordingly destabilizes and degrades the microstructure of the interconnect by such grain boundary separation and porosity. This diffusion of air electrode constituents during electrochemical operations, reduces the efficiency of the electrochemical cells and reduces the life expectancy and reliability of the electrochemical cells.
There is a need to protect the interconnect over long term operation of electrochemical cells from leaching of the constituents of the air electrode, especially manganese, into the interconnect which consequently and disadvantageously alters the microstructure of the interconnect. Any protective interlayer provided between the air electrode and interconnect must remain nonreactive, electrically conductive and chemically compatible with the air electrode and interconnect.